The present invention addresses the need, for many applications, for an alumina honeycomb monolithic support that combines both high strength and a high B.E.T. surface area (over 100 m2/g). In the past, it has been found difficult to obtain both high strength and high surface area in extruded alumina honeycomb structures without the incorporation of binder materials that reduce the purity and/or chemical activity of the alumina for catalytic applications.
The petrochemical industries currently use a variety of pellet type structures formed of gamma alumina or other oxides, e.g., pellets, pills, beads, rings, trilobes, stars, and so forth, as catalysts or catalyst support media for catalytic reactions. These structures are typically formed by extrusion from batch mixtures of alumina or other selected oxides, followed by drying and calcining. The objective is to produce shapes which are crush- and attrition-resistant when packed into reactor beds; due to their thick cross-sections and compact geometric shapes, the tensile strengths of such extruded shapes is generally not of concern. U.S. Pat. Nos. 3,969,273, 3,917,808 and 4,132,669 provide examples of aqueous extrusion batches incorporating combinations of various acids for the preparation of extruded pellets or pills of hydrated alumina, calcined (e.g. gamma) alumina, and combinations of alumina with phosphorous or other oxides.
Although technologies for making durable, active pellet-type alumina supports or catalysts are well developed, such structures are not optimally configured for most catalytic reactor applications. Pellet beds tend to exhibit relatively high flow resistance in comparison with honeycomb supports, and also develop preferential flow paths which exhaust portions of the catalyst while leaving other portions relatively unused.
Ceramic honeycombs are used in many applications in which the ceramic substrate serves simply as a physical structural support for a chemically active, high-surface-area catalyst support coating. A typical coating for these applications is a high surface area washcoating of gamma-alumina deposited on the channel walls of the ceramic honeycomb. U.S. Pat. No. 4,965,243 describes coated honeycomb structures of this type useful for automotive catalytic converters.
However, for many applications porous washcoatings are inadequate and catalysts or catalyst supports made up mostly or entirely of active, high-surface-area material must instead be used. Such applications include chemical processes wherein the kinetics of the chemical reaction(s) on the catalyst are slow relative to the diffusion and mass transfer steps involved in the overall process. An example is the hydro-desulfurization of fossil fuels in the petrochemical industry to make low sulfur gasoline and diesel fuels. Since the reaction kinetics are the slow step in such processes, it is important to provide a relatively large accessible BET catalyst support surface (more catalyst sites in a given volume) in order to allow the most effective use of reactor volume. This in turn requires that the entire volume of the catalyst or catalyst support structure be made of active, high-surface-area material, and that the pore structure of the material be such that that the reactants can diffuse in and products diffuse out of the volume of the catalyst support effectively over relatively long distances. The potential advantages of honeycomb structures of appropriate porosity and surface area in such applications include better selectivity, higher yield, lower pressure drop, lower waste or emissions, and more compact reactor designs.
Even for applications such as automotive catalytic converters, where reaction kinetics are usually not rate limiting, thinner-walled, lower mass catalyst supports are being developed to decrease exhaust gas back-pressure and improve reactor efficiency. However, with decreasing wall thickness the thermal mass contribution of the ceramic substrate relative to the gamma alumina catalyst support coating becomes an increasingly important factor that restricts the light-off speed of the reactor. A honeycomb that incorporates only active support material while dispensing with inert supporting structure will offer substantial performance advantages, and eliminate the separate and costly alumina washcoating step as well.
In order to take advantage of the potential benefits of alumina honeycombs, however, both the strength and surface area of the honeycombs must be maintained or improved. Many potential honeycomb applications require a high B.E.T. surface area for effective catalyst function, i.e., at least about 50 m2/gram, and from 150-200 m2/gram or more for some applications. High strength and good resistance to flaking are needed to maintain the structural integrity of the support in a hostile reactor environment. Higher B.E.T. surface areas mean a more compact reactor, which could lead to significant cost reductions for the overall reactor system.
In contrast to the formulations employed to provide pelletized alumina or other active oxide supports, the extrusion of honeycomb structures from simple oxides or mixtures has involved the use of supplemental bonding agents. These are typically incorporated in the extrusion batch with the oxides and remain in the fired honeycombs as permanent binders, to achieve useful flexural strength levels in the fired structures. Relatively high tensile strength, as measured by flexural modulus of rupture tests of the fired oxides or mixtures, is required to impart useful strength and durability to the fired thin-walled structures. However, such strength must be attained at low to moderate firing temperatures in order preserve the high porosity and B.E.T. surface area of the oxide starting materials.
U.S. Pat. No. 4,631,267 teaches the manufacture of extruded honeycombs of alumina, silica and titania composition that incorporate precursors for permanent silica, alumina and titania binders in powdered alumina, titania or silica extrusion batches. The precursors for the permanent binders are generally liquid solutions or dispersions of oxide-yielding compounds such as titanium isopropoxide or silicone solutions or hydrated alumina slurries, these being converted to small crystallite bonding deposits of the respective oxides on firing. Alumina honeycombs produced by this method can exhibit B.E.T. surface areas in excess of 70 m2/g and MOR (flexural modulus of rupture) strengths above 2000 psi after firing at temperatures in the 500-1000xc2x0 C. range.
Although the method of this patent provides alumina products of high surface area, it has not yet found extensive commercial application. Shortcomings of the disclosed method include the relatively high cost of the permanent binder materials, and the limited effectiveness of such binders in terms of the range of powders which can be successfully treated and the levels of fired product strength which may be obtained. Good results have been demonstrated for hydrated alumina batch powders, but the resulting batches are subject to high drying and firing shrinkages which create significant production and yield problems for fine cellular structures such as honeycombs.
What is therefore required is a method for producing alumina honeycombs that produces products of high strength and surface area, yet is still economic in terms of raw materials costs and processing yields.
The present invention is founded upon the discovery that the incorporation of relatively small amounts of inorganic or simple organic acids directly into powdered alumina extrusion batches can provide honeycombs exhibiting an excellent combination of high strength and high surface area. Moreover, the method can be used with powdered alumina batches that include substantial proportions of anhydrous high-surface-area (gamma) alumina powders, these offering a significant processing advantage in terms of reduced drying shrinkage, and therefore process yield. Strength increases of from 45-200% or more over the strengths typically attained in conventional honeycomb batches fired at the same temperature can be developed by this means.
For best retention of high surface area, the peak temperatures used to fire the extruded honeycombs should be maintained below about 1000xc2x0 C. Yet even at firing temperatures in the 500-750xc2x0 C. range, sintered alumina materials forming honeycomb structures in accordance with the invention have much higher strength, as determined by flexural modulus of rupture (MOR) testing, than similarly fired alumina formed without batch additions of selected acids.
Therefore, in a first aspect, the invention includes an improved method for making a strong extruded high-surface-area alumina-containing body such as an alumina honeycomb. In accordance with that method, a powder component for an extrusion batch is first provided, that powder consisting predominantly (at least about 80% by weight) of alumina powders and including at least one anhydrous high-surface-area alumina powder. By anhydrous alumina powder is simply meant an alumina powder that does not incorporate significant water of hydration. The anhydrous alumina portion of the powder batch should constitute a substantial proportion, i.e. at least about 40% by weight, of the dry powder batch.
The powder component of the batch is next combined with water, an acid, and a plasticizing temporary binder to form an extrusion batch. The proportions of water and plasticizing binder should be sufficient to form an extrudable plasticized batch. The acid is included in an amount at least effective to increase the ultimate fired bending strength of the final alumina body, this typically requiring at least about 1% by weight of the selected acid in the combined batch.
The combined batch is next thoroughly mixed for a time at least sufficient to achieve plasticization and good batch homogeneity, and is then extruded to form green alumina preforms of a desired shape. Complex shapes such as extruded honeycombs may readily be formed from well-plasticized mixtures, although pellets or other shapes could be formed as well.
Following extrusion, the green alumina preforms must be well dried prior to firing. Drying may comprise heating or otherwise treating the preform to accelerate water removal, although overly rapid drying must be avoided to prevent cracking of the preforms as the result of uneven shrinkage. Advantageously, the problems attending drying shrinkage in preforms produced in accordance with the invention are substantially less than encountered using hydrated gamma alumina precursors, due to the substantial proportion of non-hydrated alumina powders included in the batch.
The dried green alumina preforms are finally consolidated by firing at temperatures sufficient to bind the alumina powders into strong but porous alumina products. Since excessive firing temperatures reduce preform porosity and surface area, while insufficient heating results in a weak product, firing temperatures in the range of about 500-1000xc2x0 C. are employed. Depending upon batch composition and processing parameters, such firing can produce porous, high-surface-area products with large pore volumes and surfaces areas in excess of 100 m2/g, preferably in excess of 150 m2/g as determined by conventional BET methods. At the same time, flexural strengths of 1500-3000 psi or greater in the honeycomb wall material, as determined by conventional flexural modulus-of-rupture (MOR) bar sample testing, are provided.
Strength and surface area results obtained in accordance with the invention depend importantly on the nature of the acid selected. Large strength enhancements are observed with weak organic acids such as acetic acid; results obtained through the substitution of other acids are variable. Also important is the method of introducing the acid component into the extrusion batch. Surprisingly, adding the acid during or subsequent to the dispersion of the anhydrous alumina component in water can provide significantly higher strength in fired alumina honeycombs than normally results from the direct addition of a weak (or strong) acid component to a dry alumina mix.
The invention is particularly adaptable to the production of strong, high-surface-area alumina honeycomb catalysts or catalyst supports, since it enables the direct honeycomb extrusion of alumina batches combining major proportions of gamma alumina. The preferred batches will consist essentially entirely of alumina powders, and will include 40-100% by dry weight of non-hydrated gamma alumina in combination with the selected acid, water, and a suitable temporary plasticizing binder. Optional batch additions may include one or more surfactants, lubricants or other mixing or extrusion aids, although these are not required to obtain strong, high-surface-area products.
Commercially available gamma alumina powders constitute suitable sources of the anhydrous high-surface-area alumina batch component, with powders of surface areas above 200 m2/g being readily available. Where other porous alumina materials are to be included in the batch, commercial alumina preparations that, upon calcining, provide gamma-alumina or other transition aluminas of high surface area can be used. Examples of the latter include commercially available boehmite and pseudo-boehmite powders. The particle sizes of the alumina materials employed in formulating the batches are not critical, but may be adjusted as desired for the purpose of modifying the internal pore size and size distribution of the product.
The inclusion of a temporary binder in the honeycomb formulations of the invention is helpful to improve the plasticity of the compounded batch for better extrusion characteristics. By a temporary binder is meant a binder that is substantially completely burned off at the temperature at which the honeycomb is fired. The temporary binder can be any of the well-known materials routinely used in the ceramic arts for such purposes. Common examples include the cellulose ether binders such as methyl cellulose, commercially available as Methocel cellulose ether products from the Dow Chemical Co.
The weight proportions of alumina, water and temporary binder and acid may be adjusted as necessary to obtain a consistency and plasticity desirable for extrusion, together with a xe2x80x9cgreenxe2x80x9d strength adequate for handling the honeycombs after forming but before firing. Generally, 1-10 parts by weight of the selected temporary binder are added for each 100 parts by weight (dry) of alumina powders. Surfactants, if present, will normally not exceed about 2 parts by weight per 100 parts of alumina powder, at least where a conventional extrusion aid such as sodium stearate or stearic acid is used. Water is included in proportions necessary to provide a plastic extrusion batch, typically in amounts between about 40-90% by weight of the total batch weight of dry and wet ingredients.
As noted above, the acid selected for incorporation into the combined batch can have a significant effect on the properties of the final alumina product. The use of acetic acid is particularly preferred, although other short-chain organic acids such as formic acid can offer a somewhat smaller strength enhancement benefit. Satisfactory results are also expected from monovalent mineral acids such as HNO3 and perhaps HCL, if used in concentrations (in milliequivalents of acid per 100 g alumina) similar to those used in acetic acid-containing batches. The latter batches typically comprise at least 1% by weight of acid in the combined (wet) batch, more preferably about 1-5 parts concentrated (99-100%) acetic acid for each 100 parts of anhydrous alumina by weight. On the other hand, the strength improvements observed with higher molecular weight organic acids such as citric acid and oleic acid appear to be much less significant.
Prior to introducing the liquid and acid components into the batch, it is desirable that the dry batch components be first thoroughly mixed, for example, in dry blending equipment such as a Littleford(trademark) mixer. Thereafter, the blended dry batch can be transferred to batch blending and plasticizing apparatus such as a mix-muller for combination with the water and acid ingredients of the batch. In general, best combined batch quality is obtained by first adding the acid to the batch water, and then adding a well mixed dry blend of the alumina, temporary binder and optional extrusion aids to the mix, with continued blending until a homogeneous plasticized mass is obtained. If desired, the plasticized batch produced by the muller or other mixer may then be pre-extruded through a spaghetti die one or several times, to complete the mixing process and remove any air inclusions from the mixture prior to final forming.
Forming of the plasticized batch into alumina honeycombs or other products can be carried out utilizing ordinary extrusion equipment together with any of the known ceramic honeycomb dies employed for honeycomb extrusion in the prior art. The handling characteristics of the alumina batches provided according to the invention are such that a relatively wide range of honeycomb geometries can readily be produced. Honeycombs having cell wall thicknesses in the range of 0.1-2 mm and cell densities in the range of 10-600 cells/in2 of honeycomb cross-section can be formed with commercially available ceramic ram or screw extruders and ceramic honeycomb extrusion dies of appropriately selected dimensions and cell shapes.
Extruded alumina honeycomb shapes produced as above described may be dried in accordance with practices conventional for the production of ceramic honeycombs. Advantageously, however, the rate of drying can be somewhat more rapid than in the case of conventional alumina honeycombs since the risks of cracking and shape distortion are correspondingly reduced. This is because the drying shrinkages observed with these batch mixtures can be 50% or more below those typically observed in honeycombs produced principally from hydrated alumina materials (alumina monohydrates and trihydrates such as commercial boehmite materials). Depending on the amount of hydrated material included in the batch, the latter can exhibit linear drying shrinkages as high as 18-25%.
Firing of the dried honeycombs is carried out at relatively low temperatures, generally in the range of 500-1000xc2x0 C. With the alumina batches provided as above described, strong bonding of a honeycomb support structure or catalyst without undue surface pore consolidation and/or loss of internal wall porosity can readily be achieved at these temperatures. Most advantageously, strong products are produced without resort to permanent binders, or the use of other bonding strategies relying on supplemental bonding additives such as used in the prior art to provide high strength, high-surface-area structures.
The firing shrinkages demonstrated by the dried green products vary depending upon the presence or absence of hydrated alumina from the batch, as well on the peak firing temperature employed. In general, however, linear firing shrinkages are relatively small (on the order of 3%) and do not unacceptably degrade final product surface area and wall porosity.